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Cotton leaf Spot

Cotton is blooming now in many fields and there are a good many sprays going out to protect it from Target spot (Corynespora). I have not seen any yet this year and I talked to Crop Consultant Wes Briggs and he said he has not seen any either but conditions are right so we are on the lookout. I have an old photo below.

1) Conditions are perfect for target spot (Coprynespora). 2) Effective timing requires application before a
complete canopy develops and before disease is established. 3) From our limited data, the single BEST timing last season for management (not control..) was the 3rd week of bloom. The best results were from the applications at
1st week of bloom followed by a second application at the 3rd week of bloom.

Here’s an old photo of some of the leaf spot.

As in 2010 and 2011, most, if not all, of the cotton crop across the Coastal Plain was affected last season by leaf spots that caused significant premature defoliation in many fields. In mild cases the diseased spots were a curiosity in the field; in severe cases complete defoliation occurred across large areas of a field. Extreme heat and drought across Georgia in 2011 greatly increased the symptoms of Stemphylium leaf spot, a disease which is closely tied to a deficiency of potassium in the cotton foliage. Rainfall was more abundant across the Coastal Plain in 2012 and temperatures were cooler as well. Though such conditions reduced the overall importance of Stemphylium leaf spot, target spot, caused by Corynespora cassiicola, was abundant not only in Georgia, but also in states from Mississippi to Virginia. Considerable research was conducted in Georgia last season for the management of target spot and fungicides were effective in reducing symptoms associated with the disease. As this section is being completed work is being completed to compile data regarding yield loss to target spot in Georgia. However reports from Virginia document that use of fungicides increased yield by as much as 200 lb lint/A; reports from Alabama document yield increases of 200-700 lb seed cotton/A where fungicides were applied.

Rainfall and irrigation help to spread target spot in at least two ways. First, rain-splash helps to move spores of the target spot pathogen from the debris on the soil to the lower leaves of the cotton plant, where infection occurs leading to the production of leaf spots. Second, rainfall and irrigation provide the moisture needed for spore germination and infection to occur. Moisture is also important for the production of spores on the spots and for their dispersal and infection of new tissue. Rainfall and irrigation are critical for the production of cotton in Georgia; however anything that increases periods of leaf wetness, to include dew, will facilitate the development of target spot. Although management of leaf spot diseases will be discussed elsewhere in this section, below are factors that UGA Cooperative Extension believes increases the risk of a cotton crop to target spot. As the risk to target spot increases, the potential benefits to use of a fungicide to protect yield also increase.

Factors that likely increase risk to target spot of cotton:

1. Cotton planted in short rotation, especially in fields where target spot has been a problem in the past.

2. Rank growth in the field, either because of management of other factor, e.g., variety.

3. Field receives overhead irrigation.

4. Rainfall abundant or excessive in a growing season.

Other factors that could contribute to increased risk to target spot include variety selection (research is being conducted now to assess such) and reduced tillage systems (that may allow spores of the fungal pathogen to survive in the crop debris and to be more readily dispersed to the new cotton crop through rain splash or irrigation).

Factors to consider for use of fungicides for the management of target spot:

1. An exact fungicide program has not been established for the management of target spot; however an effective program will likely include 1-2 applications of an effective product.

2. The timing of the first spray will likely vary based upon weather conditions during a season; however considerations for timing will include:

a. Increased risk to the disease (as assessed above).

b. Detection of small amounts of disease in the field, before the disease has become established and certainly before significant defoliation has occurred. (Note: Best management for any plant disease is achieved by protecting the crop BEFORE disease is established in the field. Because we still have much to learn about target spot and because there are many fields that may not respond to use of fungicides, growers may choose to wait to see if the disease can be found in their field. Such a “wait-and-see” strategy requires careful scouting to assure success.)

c. Initiation of the fungicide program before the canopy of cotton foliage closes in order to allow for appropriate coverage of the leaves.

3. Our current recommendation for initiating a fungicide application on cotton for target spot is when the crop is between 1 and 3 weeks after first bloom. Depending on conditions, e.g., wetter or drier, the optimal time for beginning a program could change.

4. Growers should begin to assess the need for a second application of fungicide no earlier than 3 weeks after the first application.

It is currently unclear if some varieties of cotton are more susceptible to target spot than are other varieties; however work continues to answer this important question. Regardless of variety, the severity of target spot can be minimized by managing cotton growth with PGRs to eliminate rank growth.